A lithium-ion battery has been widely used as a battery for portable instruments, notebook-sized personal computers, electric vehicles, and hybrid vehicles. A lithium-ion battery normally has excellent capacity and energy density, and LiCoO2 has been mainly used as the cathode material for a lithium-ion battery. However, since cobalt is a rare resource, use of LiMnO2 and LiNiO2 as the cathode material has been extensively studied.
In recent years, LiFePO4 has attracted attention as an alternative material, and has been extensively studied. Since Fe is an abundant resource, and LiFePO4 exhibits excellent high-temperature properties in spite of relatively low energy density, LiFePO4 has been expected to be a cathode material for an electric vehicle lithium-ion battery.
However, since LiFePO4 has relatively low electron conductivity, lithium vanadium phosphate (Li3V2(PO4)3) that includes V instead of Fe, and has a NASICON (Na super ionic conductor) structure, has attracted attention.
It has been known that lithium vanadium phosphate may be produced by mixing and grinding a lithium source, a vanadium compound, and a phosphorus source, forming the resulting homogenous mixture into pellets, and calcining the pellets (see Patent Documents 1 and 2, for example). Patent Document 3 discloses a method in which vanadium oxide is dissolved in an aqueous solution that contains lithium hydroxide, a phosphorus source and carbon and/or a nonvolatile organic compound are added to the solution, the resulting raw material mixture is dried to obtain a precursor, and the precursor is heated in an inert gas atmosphere to obtain a composite of Li3V2(PO4)3 and a conductive carbon material.